1. Field of the Invention
The present invention relates to an image forming apparatus having an image deviation correction function capable of forming images on both sides of a sheet of paper by a printer or a copier.
2. Description of the Related Art
There is an image forming apparatus for forming images on both sides of a sheet of paper by an electro-photographic method such as a copier or a printer. In such an image forming apparatus, an image forming operation is performed on the first side of a sheet of paper, and then the sheet of paper is turned upside down and is conveyed again to the transfer portion, and the image forming operation is performed on the second side.
On the other hand, the sheet of paper, since water evaporates by heating at the time of fixing during the image forming operation, shrinks in size. The thermal shrinkage of the sheet of paper, when time elapses, is restored almost to its original size. Therefore, a toner image is heated, pressurized, and fixed on the first side of the sheet of paper, and then the sheet of paper is stacked once on the stack until the sheet of paper is restored to its original size and is conveyed again, and the image forming operation is performed on the second side, thus there is no fear of an occurrence of an effect of image deviation due to thermal shrinkage on the first and second sides.
However, at the request of speed-up and miniaturization of the image forming apparatus, when obtaining images on both sides, the time from execution of the image forming operation on the first side of a sheet of paper to execution of the image forming operation on the second side by re-conveying is required to be shortened. Therefore, in recent years, an apparatus for performing the image forming operation on the first side of a sheet of paper, then turning over the sheet of paper during passing on the re-conveying path without stacking it once on the stack, and immediately re-conveying it to the transfer portion has been put into practical use.
However, in such a high-speed double-side image forming apparatus, a sheet of paper thermally shrunk due to fixing during image forming on the first side, before restoration to its original size, is subject to the image forming operation on the second side by re-conveying. As a result, when the sheet of paper is restored to its original size after the image forming operation on the second side of the sheet of paper by re-conveying, between the formed image on the first side and the formed image on the second side by re-conveying, an image deviation due to thermal shrinkage is caused.
The image deviation between the first side and the second side of the sheet of paper is caused not only by thermal shrinkage of the sheet of paper but also by the re-conveying accuracy of the sheet of paper in the re-conveying path. Furthermore, due to the dimensional accuracy of a sheet of paper to be used, an image deviation is caused between the first side and the second side of the sheet of paper.
Therefore, conventionally, in Japanese Patent Published Application 2000-305324, an image forming apparatus of a high function for forming marks in the four corners of the first side of a recording medium, after fixing, reading the position relationship of the edges and marks of the recording medium by a line sensor, and from the read results, correcting a deviation between the main scanning direction and the sub-scanning direction of the second side is disclosed. However, in this image forming apparatus, to detect the mark position, an expensive line sensor must be used, disturbing a decrease in cost.
Furthermore, conventionally, in Japanese Patent Published Application 2003-241610, an image forming apparatus for, during conveying a transfer material to the detector twice, measuring the length of the transfer material before passing the fixing roller and the length of the transfer material after passing the fixing roller and from the changing amount, correcting a deviation between the main scanning direction and the sub-scanning direction of the second side is disclosed. However, in this image forming apparatus, the changing amount in the length of the transfer material is measured and from the results, a deviation between the main scanning direction and the sub-scanning direction of the second side is corrected. Namely, regarding the main scanning direction, the changing amount in the width of the transfer material is not measured, so that the correction of the deviation in the sub-scanning direction is applied as it is. Therefore, in the main scanning direction, the deviation correction accuracy is reduced. Furthermore, the center position deviation of the transfer material caused by the accuracy at the time of re-conveying of the transfer material cannot be measured, thus the deviation correction accuracy in the main scanning direction is reduced.
Therefore, in an image forming apparatus for executing the image forming process on both sides of a sheet of paper, an image forming apparatus having an image deviation correction function for, although at a low price and easily controllable, preventing an image deviation in the main scanning direction caused on the first and second sides of the sheet of paper and easily obtaining double-side images of high quality is desired.